This study is a stage 1b behavioral therapy development grant exploring the use of Mindfulness Based Relapse Prevention (MBRP), an efficacious eight session group therapy for the treatment of individuals with substance use disorders, in women with comorbid PTSD and SUD enrolled in community substance abuse treatment. The MBRP manual will be adapted to address PTSD, specifically emotional dysregulation and reactivity. PTSD symptoms contributes to the onset and maintenance of substance use so addressing both disorders in an integrated fashion is indicated for successful outcomes. Women with comorbid PTSD and SUD represent a large proportion of patients treated in community substance abuse treatment. The revised manual will be piloted in a randomized control study with 80 women enrolled in women's intensive outpatient treatment. PTSD symptoms severity, substance use outcomes and measures of mindfulness and emotional regulation will be assessed at baseline, weekly throughout the intervention, post intervention and at 3-, and 6- month follow- ups. Women will be randomized to either MBRP plus treatment as usual (TAU) or TAU alone in a 2 (MBRP) to 1 (control) randomization schedule. The MBRP group will replace a trauma focused group routinely conducted in TAU. Outcomes process evaluation during the course of the project will guide the refinement of the manual to address issues related to PTSD. This proposed study addresses several of the program announcement specific areas of interest: 1) drug abuse treatment interventions, including interventions for patients with comorbidities, in diverse settings, 2) understanding the essential components and mechanism of behavior change using MBRP intervention, 3) treatment training and supervision to include procedures for adapting the intervention and fidelity measures for feasibility to be disseminated to community providers. Patient satisfaction and provider acceptability, feasibility and ease of implementation will be monitored to increase the potential for program adoption. If data gathered in this project is promising with an effect size estimation obtained, a larger randomized controlled study will be developed.